Electrodeposition of tin



7 Patented Aug. "31,- 1926.

UNITED s'rATEs PATENT orrlca.

noitncn auss'nm. McILHENNEY, or runway, new. mnsnx, assrenoa 'ro vuncm narnmme comrm, or snwnnarr, NEW Jnnsmr, a conrom'rron or new JERSEY.

No Drawing.

This invention .relates to the electrodepo- "sition of tin and has for its objebt an 1mprovement in the electrodeposition of the tin. v Y I Tin is eletro'de osited by passing a direct current of electncity through anappropriate eletrolyte such as a solution of a tin salt or compound. The electric current passes from one pole or electrode called the anode through the electrolyte to the other pole or electrode'called the cathode. As soon as this flow of electric current starts, all the anions in the solution. begin to move toward the anode, while all the cations begin to move to- 1 ward the cathode. In the "case of a tinelectrolyte, the metallic tin ions comprise at least part of the cations, and in the course of the el'ectrodeposition they are deposited as metallic tin in the cathode. The anions, and) in the case of certain electrolytes some of the cations, may escape as gas, or may be de-' posited on one of the electrodes in some form or other, or may react with the electrolyte to form new compounds that remain in the electrolyte.

Various salts and compounds of tin may be used in preparing the tin electrolyte or electroplati ng bath. Thus, in commercial practice an aqueous solution of tin chloride (stannous chloride) or an aqueous solution of sodium stannate are frequently, used as electrolytes, As a direct current is assed throughsuch an electrolyte, the metaLic tin deposits on the cathode and the chlorine ions composition of the tin-bearing compounds present in the bath. V I e QEWWWQ 9 39' tiu'bearing acids, alkalies, or salts.

or sodium ions, as the-case may be, reactdeposition. electroplating baths, other than those using ELECTRODEPOSITION OF TIN.

Application filed April 14, 1926. Serial No. 101,921.

pound may be made which, though not appreciable soluble in water, is soluble inand combines with certain acids, such as hydrochloric acid; certain alkalies, such as sodium hydroxide; and with certain salts, such as sodium cyanide, when said acids, alkalies or salts are in a ueous solution. This tin bearing compoun acid or alkali whenused to form, or as a component of, certain tin electroplating baths, but it does introduce tin into these electroplating baths by reason of its co1n position.

By virtue of the property of this-tinbearingcompound of dissolving in the aqueous solution of certain acids, such as hydrochloricacid, certain alkalies, such as sodium hydroxide and certain salts, such as sodium cyanide, andcombining therewith, it is suitable for preparing certain .tin electroplating baths of a desired initial tin content, withdoes not, of itself, introduce.

in the limits of its solubility, by the addition of this tin-bearing compound to baths containing definite known quantities ofsuch This tin-bearing compound is also suitable for addition, either in excess at the start or adually during the progress of the electro eposition, to certain tin electroplating baths wherein certain acids, such as hydrochloric acid, certain alkalies, such as sodium hydroxide and certain salts, such-as sodium cyanide, are formed by the decomposition of the components of these baths in the course of the electrolysis. Thus,

this tin-bearing compound is suitable for additi'on to tin electroplating baths prepared 'from-such tin-bearing compounds as an alkali metal stannate (sodium stannate), wherein an alkali metal hydroxide, such as sodium hydroxide, is formed due to the decomposition of the alkali metal stannate by the removal of tin from, the bath by electrog able for addition to tin electroplating baths prepared from such tin-bearing compounds as. stannous chloride, wherein an acid, hydrochloricacid, is formed due to decomposition of the stannous chloride by the removal of tin from the bath by electrodeposition.

A tin-bearing compound havingthe prophis tin-bearing compound is suit-.

erties hereinbefore described may be prepared by treating the aqueous solution of a or other appropriate means and is then preferably washed with water to remove soluble admixed salts resulting from the neutralization, whole or partial, of the alkali of the stannate. The precipitate is not dried. since upon drying its'solubility in such acids as hydrochloric acid, such alkalies as sodium hydroxide and such salts as sodium cyanide is lowered.

The tin-bearing compound, prepared as described, is practically neutral, in itself, and is only slightly soluble in water. It is. however, readily soluble in acids, such as hydrochloric acid, and in alkalies,.sueh as sodium hydroxide, even in dilute solution, and reacts with these acids and alkalies to form definite 'chemical compounds. Thus, with an acid, such as hydrochloric acid, a chloride of tin results, and with an alkali. such as sodium hydroxide, sodium stannate is formed. The soluble tin-bearing compound as prepared for use in the practice of this invention is a white, amorphous, semi-plastic mass, containing approximate ly 34% metallic tin and about 46% water.

The stannate solution from which the tinbearing compound is made may be of any concentration, but is preferably used in an undersaturated condition. When an acid is used as a precipitant, it may be diluted with water, say an equal volume,- although the concentration of the acid is not of primary importance. The amount of acid to be added, however, is that which will be required for neutnalization of the alkali of the stannate. and this may be determined by analysis of a test portion of the stannate solution. The calculated amount of acid is then added to the stannate solution while the latter is kept agitated and preferably heated.

lVhen an acid salt is used as the precipitant, such a slat is picked out as will break up under the application of heat and form a gas of acid character which in turn will react on the sodium stannate. The amount of such salt to be used will depend on the amount of acid gas which itwill release under the application of heat and the quantity of sodium stannate present.

When an acid salt, such as sodium bicarbonate. is used as the precipitant, it is preferably dissolved in water, either hot or cold,

and this solution is then added to the stannate solution. The intimate mixture of the two solutions, preferably by agitation, is

brought about and the sodium bicarbonate is decomposed by heating the mixed solutions. The decomposition of the sodium bicarbonate brought about by heating the solution causes the liberation'of carbon dioxide which. combining with the alkali of the stannate. precipitates the tin-bearing compound. The amount of sodium bicarbonate required to treat a batch of stannate solution is based upon the carbon dioxide which will be liberated upon decomposing the bicarbonate by means of heat.

When a gas of acid character such as carbon dioxide, is used as the precipitant, the gas is brought into direct contact with the stannate solution. During the reaction with carbon dioxide, the stannate. solution is preterably heated. The amount of carbon dioxide required to secure complete precipitation is determined by the quantity of alkali stannatepresent, since this reaction takes place only between the alkali of the stannate and the carbon dioxide. The quantity of carbon dioxide is determined by an analysis'of a test portion of the stannate solution.

A specific example of a tin electroplating bath, in which the use of the tin-bearing compound, prepared as above described. represents an improvement over established practice, is that of a bath formed by solution of sodium stannate in water. As tin is electrodeposited from such a bath, sodium hydroxide is formed and, with continued electrolysis, accumulates in the bath to such an extent as to render the bath less etlicient and ultimately unfit for further use, even though it may still contain a considerable amount of tin in theform of sodium stannate. A sufiicient addition of the tinbearing compound, prepared as herein de scribed, to such an electroplating bath will restore the soluble tin content to normal and reduce the sodium hydroxide content,

by causing the sodium hydroxide to react" nate bath may be sustained and the exressive accumulation of free sodium hydroxide prevented.

In practice the tin-bearing compound, prepared as herein described, may be added in excess to the sodium stannate bath as the latter is made up and the compound remains unafiected in the bath until electrode ition begins. Then, as tin is deposite on .tobead the cathode and sodium hydroxide formed simultaneously in the bath, the-'tin-bearing compound dissolves in and reacts with the sodium hydroxide so formed to produce sodium stannate. The cycle of tin deposi-. tion, simultaneous sodium hydroxide formation and solution of the tin-bearing compound to form sodiumstannate proceeds as described as long as the bath isoperated.

The weight of the tin-bearingcompound suflicient metallic tin to the bath to compensate for theweight of tin which will be de- .posited from the bath during any given time. If desired, the tin-bearing compound may be added to a sodium stannate bath gradually during its use, so that an undissolved .excess of the compound is present at all times. 4

In the sodium stannate bath, described above, the anodes may be formed of iron or other material of suitable shape-and size and the cathode formed by the article 01. articles to be electroplated.

Besides the example given, where the tinbearing com undis used with a sodium stannate bat already formed, a sodium stannate bath may be formed initiall from the tin-bearing compound, by dissolving calculated amounts, based on its. tin content,

of the tin-bearing. compound'in sodium hydroxide or by the addition of an excess, i. e., more than will dissolve therein, to a calculated amount of sodium hydroxide,

I thus yielding a sodium stannate electroplatmg bath of any desired initial tin content,

which may thereafter be sustained, during electrolysis, by keeping an undissolved excess of the tin-bearing compound present at all times.

Where the tin-electrolyte is stannous chloride in solution, the reaction during the ,electrodeposition ,of the tin results in the formation in the electrolyte of hydrochloric acid. The tin-bearing compound, prepared as herein described, may be added to this electrolyte in the same manner as hereinbefore described in connection with a sodium stannate electrolyte. The hydrochloric acid in the stannous chloride electrolyte reacts with the tin-bearing compound to form water soluble tin chloride, and thereby the tin content of the electrodeposition bath is maintained. and excessive accumulation of hydrochloric acid prevented. If desired the tin electrolyte maybe initially formed by the solution of the tin-bearing compound in hydrochloric acid.

The invention, whileof particular advantage in electroplatlng tm, may be applied to I any method of tin electrodeposition, such,

ed toa sodium stannate bath should preferably be that which will supply for example, as the production of pig-tin by electrolysis and subsequent smelting."

I claim:

1. The improvement in the e'ectrodeposi tion of tin employing an appropriate electrolyte, which comprises supplying tin to the electrolyte by the introduction thereto of a compound of tin that is substantially insoluble in Water but soluble in the products of the electrolysis forming in the elecrol'yte as electrodeposition proceeds.

2. The improvement in the electrodeposi-- tion of tin from an appropriate electrolyte,

which comprises supplying tin to the electrolyte by the introduction thereto of a compound of tin that is substantially insoluble in water but soluble in the electrolyte as electrodeposition proceeds, said compound of.

tin of itself supplying no acid on alkali component when introduced into the electrolyte.

3. The improvement'in the electrodepos'ition of tin from an appropriate electrol e, I

which comprises supplying tin to a the e cotrolyte by the gradually regulated introduction thereto of a compound of tin that is substantially insoluble in water but is-soluble in and combines with the electrol and the lproducts of electrolysis associate therewit a 4. The improvement in the electrodeposition of tin from an appropriate electro yte, which comprises maintaining in the electrolyte an excess of a tin-bearing compound that issubstantially insoluble in water but soluble in the electrolyte and the products of the electrolysis associated therewith.

.. 5. The improvement in the electrodeposition of tin, which comprises reparing a tin-- of tin of-itself supplying no acid or alkali component to the electrolyte. v 1 7. The improvement in the eleetrode'position of tin from an electrolyte containing an alkali metal stannate, which comprises maintaining a fixed pro ortion of tin in solution in the electrolyte y adding thereto a tinbearin compound substantially insoluble in water ut soluble in the solution of alkali metal hydroxide formed in the electrolyte as tin is electrodeposite therefrom. i

8. The improvement in the electrodeposition of tin, which comprises preparing a tinbearing electrolyte by dissolving in an appropriate alkali metal hydroxide, such as sodium hydroxide, a compound of tin which is substantially insoluble in water but soluble in solutions of hydrochloric acid, sodium hydroxide, and sodium cyanide.

9. The improvement in the electrodeposition of tin from an appropriate electrolyte, which comprises supplying tin to the electrolyte by the introduction thereto from time to time of a compound of tin that is substantially insoluble in water but soluble in the electrolyte and the products of electrolys'is associated therewith, said compound of tin supplying of itself no acid or alkali component to the electrolyte.

10. The improvement in the electrodeposi- 

